1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for deghosting seismic data recorded with land buried multi-component seismic receivers.
2. Discussion of the Background
During the past years, interest in monitoring oil and/or gas reserves has increased. Time-lapse (or 4D) seismic monitoring of producing oil fields is an accepted method for optimization of field development and product recovery, providing significant improvements in recovery rates and savings in drilling costs.
Time-lapse seismic reservoir monitoring is the comparison of 3D seismic surveys at two or more points in time. Time-lapse seismic reservoir monitoring also has potential for increasing ability to image fluid movement between wells. A traditional configuration for achieving a 4D seismic monitoring is illustrated in FIG. 1. FIG. 1 shows a system 10 for the acquisition of seismic data. The system 10 includes plural receivers 12 positioned over an area 12a of a subsurface to be explored and in contact with the surface 14 of the ground. A number of vibroseismic sources 16 are also placed on the surface 14 in an area 16a, in a vicinity of the area 12a of the receivers 12. A recording device 18 is connected to the plurality of receivers 12 and placed, for example, in a station-truck 20. Each source 16 may be composed of a variable number of vibrators, typically between 1 and 5, and may include a local controller 22. Alternatively, the source may be a shallow buried explosive charge or other known devices for generating a seismic source, e.g., a metal plate placed on the ground and hammered with a hammer. A central controller 24 may be present to coordinate the shooting times of the sources 16. A GPS system 26 may be used to time-correlate the sources 16 and the receivers 12.
With this configuration, sources 16 are controlled to generate seismic waves, and the plurality of receivers 12 record waves reflected by the oil and/or gas reservoirs and other structures. The seismic survey may be repeated at various time intervals, e.g., months or years apart, to determine changes in the reservoirs. Although repeatability of source and receiver locations is generally easier to achieve onshore, the variations caused by changes in near-surface can be significantly larger than reservoir fluid displacement, making time-lapse 4D seismic acquisition and repeatability challenging. Thus, variations in seismic velocity in the near-surface are a factor that impacts repeatability of 4D surveys.
Thus, an aim for reservoir monitoring is to measure velocity variations in the subsurface by measuring small differences in the first arrival times of the refracted waves. These arrival time changes are typically measured using cross-correlations, which are biased by velocity changes in the near-surface layer. When using a shallow buried recording system, the waves arriving at the recorder (up-going energy) are not biased by velocity changes in the rocks above. However, the waves that reflect from the earth's surface back into the earth are affected. Typically both wave fields are recorded within close temporal proximity and their signals interfere. This makes it difficult to extract the unbiased information that is provided by the up-going energy. When the two wave-fields are separated, it is possible to measure velocity changes in the layers below the receivers that are not affected by velocity changes in the near-surface layer above the buried receivers.
Thus, up- and down-going energies may be used for monitoring the reservoir. Various algorithms for separating up-going and down-going energies are known. However, these algorithms are affected by the velocity of the ghost in the layer between the earth's surface and the burial horizon of the receivers for the case of buried receivers. The change in the velocity, spatially and/or over time, affects the delay time for the ghost and, thus, may cause problems for the algorithms that “pick” the first arrival times.
Thus, there is a need for developing a device and a method for better deghosting the recorded seismic energy.